1. Field of the Invention
The present invention relates to a monolithic ceramic capacitor and a method of producing it. More particularly, the present invention relates to a monolithic ceramic capacitor having dielectric ceramic layers each made of a ceramic material comprising strontium titanate as the main component and bismuth oxide or an oxide of a bismuth compound as an additional component and containing a reduction inhibitor, and also to a method of producing it.
2. Description of Related Art
A monolithic ceramic capacitor is generally composed of a plurality of dielectric ceramic layers with inner electrodes sandwiched between the adjacent ceramic layers and with outer electrodes disposed on the outer surface of the laminate and connected with each inner electrode.
Such a monolithic ceramic capacitor may be produced according to the following process. A dielectric powder is mixed with an organic binder; the resulting mixture is made into a slurry; the slurry is shaped into green sheets by a doctor-blading method; the sheets each are printed with a paste containing a metal powder by a screen-printing method or the like; a plurality of the green sheets each printed with the paste are laminated; the resulting laminate is subjected to pressure to establish tight connections between the green sheets; the laminate is then fired in air at from about 1300.degree. to 1400.degree. C.; an outer electrode material is applied onto the surfaces of the sintered body having exposed inner electrodes; and the material is baked.
Dielectric ceramic layers consisting essentially of strontium titanate and containing a bismuth compound give the monolithic ceramic capacitor a relatively large dielectric constant with small voltage dependence and a small dielectric loss and therefore high resistance to high voltage.
A material of the inner electrodes comprising a noble metal such as platinum, gold, palladium or their alloys is advantageous in that (1) the inner electrode material does not react with the dielectric ceramic during firing since the noble metal has a melting point higher than the temperature at which the dielectric ceramic is fired and that (2) the inner electrode material is not oxidized even when fired in air. However, an electrode material comprising such a noble metal is expensive. For example, the cost of the electrode material may often be from 30 to 70% of the total material cost of the ceramic capacitor.
Moreover, an inner electrode material comprising a silver-palladium alloy often worsens the characteristics of the capacitor. For example, the dielectric constant of the capacitor is often lowered and becomes unstable due to the migration of silver.
In order to solve these problems, base metals such as nickel, copper, iron, cobalt, tungsten, molybdenum, etc. could be used as the inner electrode material. However, the material comprising such a base metal must be fired in a neutral or reducing atmosphere in order to prevent the oxidation of the material during firing. If fired in such conditions, the bismuth compound in the dielectric ceramic layers is reduced and, as a result, the dielectric ceramic becomes semi-conductive. Accordingly, a ceramic capacitor would not be produced.
Copper can be fired at about 1000.degree. C. in a neutral atmosphere having an oxygen partial pressure of 10.sup.-7 MPa but it is easily oxidized at relatively low temperatures. An inner electrode material comprising copper also diffuses copper into the dielectric ceramic layers during firing at an oxygen partial pressure that is higher than the equilibrated oxygen partial pressure of Cu/CuO and often lowers the characteristics of the capacitor produced. Therefore, an inner electrode material comprising copper requires the strict control of the oxygen partial pressure in the firing atmosphere, which, however, results in the increase in the production cost.